I guess it completely depends on how much you shoot and what, but I think a 135 format camera and a micro 4:3 complements each other well. Something like a 5DN brings continuous auto-focus, shallower depth of field, a bit less noise at both extremes of the ISO range and different ergonomics. But it does so at the cost of size, weight and money. You have to figure out how much use you'll get out of those advantages, before you can decide if it'll be a good investment or a waste of money.

Personally, I don't miss having a larger dSLR. Though I envy the advantages of a larger sensor, the smaller size and weight more than makes up for it. A 5D3 or D800, good as they may be, would sit in my cupboard collecting dust. Your mileage may vary.